1. Field of the Invention
The present invention relates to the use of specific high Group VIII metal content crystalline aluminosilicate zeolite catalyst in a vapor phase isomerization process, said catalyst comprising a crystalline aluminosilicate zeolite characterized by a silica/alumina mole ratio of at least 12 and a constraint index, hereinafter defined, within the approximate range of 1 to 12.
2. Description of the Prior Art
The catalytic rearrangement of alkyl groups present in alkyl aromatic hydrocarbons to provide one or more products suitable for use in the petroleum and chemical industries has heretofore been effected by a wide variety of catalysts. Acidic halides such as aluminum chloride, aluminum bromide, boron trifluoride -- hydrogen fluoride mixtures, etc. have been used in the rearrangement of alkyl benzenes to provide valuable intermediates which find utility in the synthesis of rubber, plastic, fibers and dyes. Other catalysts which have been used include solid siliceous cracking-type catalysts such as silica-alumina and clays and platinum deposited on silica-alumina. Although various catalysts possess one or more desired characteristics, a majority of catalysts heretofore employed suffer from several disadvantages. Acidic halides such as aluminum chloride, for example, are partially soluble in the feed material and are easily lost from the catalyst zone. Catalysts of this type are also uneconomical because of their extreme corrosiveness and requirement for recovery from the effluent products. Other catalysts of the heterogeneous type, such as silica-alumina, platinum on alumina, etc., do not possess sufficient acidity to provide effective conversion and necessitate the use of relatively high temperatures above the order of 800.degree. to 950.degree. F. High temperatures frequently lead to coke formation which lowers the yield of desired product and necessitates frequent regeneration of the catalyst to remove coke. This results in reducing on-stream time and leads to high catalyst consumption due to loss of catalyst activity. Heterogeneous catalyst such as the crystalline aluminosilicates, both natural and synthetic, possess sufficient acidity but suffer the disadvantage or poor selectivity and aging as evidenced by "coke" make and the excessive amounts of disproportionated product formed in isomerization reactions.
A process in the art for isomerization of xylene is Octafining, extensively discussed in the literature as exemplified by:
1. Pitts, P. M., Connor, J. E., Leun, L. N., Ind. Eng. Chem., 47, 770 (1955).
2. Fowle, M. J., Bent, R. D., Milner, B. E., presented at the Fourth World Petroleum Congress, Rome, Italy, June 1955.
3. Ciapetta, F. G., U.S. Pat. No. 2,550,531 (1951).
4. Ciapetta, F. G., and Buck, W. H., U.S. Pat. No. 2,589,189.
5. Octafining Process, Process Issue, Petroleum Refinery, 1st Vol. 38 (1959), No. 11, Nov., p. 278.
The catalyst for use in such process is platinum on silica-alumina.
An improved catalyst for use in Octafining plants is taught by U.S. Pat. No. 3,856,872 to be of the ZSM-5 type of zeolite, whereby the process operates at high space velocities.
Even in such an improved process, especially when the catalyst has increased acid activity resulting from decreasing intracrystalline diffusional resistance, there is a loss of xylene presumably due to disproportionation of xylenes and transalkylation of xylenes with ethylbenzene. Metal of Group VIII of the Periodic Table of Elements is incorporated in the ZSM-5 containing catalyst primarily as a hydrogenation component that in the presence of hydrogen will inhibit coke formation and reduce aging.
It is hereby proposed and demonstrated that by increasing the Group VIII metal content of such a catalyst to an established minimum, improved balance between the hydrogenation activity of the Group VIII metal and the acid activity of the HZSM-5 permits greater hydrodealkylation, particularly deethylation of the C.sub.9.sup.+ fraction formed. Since a good portion of the C.sub.9.sup.+ fraction is dimethylethylbenzene, formed by the transalkylation of xylene with ethylbenzene, deethylation of this material would result in a recovery of product in the xylene fraction previously lost in the C.sub.9.sup.+ fraction. Furthermore, the improved deethylation activity of the catalysts with increased Group VIII metal content will increase the ethylbenzene disappearance at the same severity with the concomitant advantage of higher value benzene byproduct rather than the heavy polyethylbenzenes.